Bug#941198: In support of mandatory unit files

[Guillem Jover <guillem@debian.org>]

On Sun, 2019-12-08 at 11:15:57 -0800, Russ Allbery wrote:
> > Sure, help fir that would be nice. Thanks for the offer.  (Probably
> > should have an own bug for that.) Nethertheless, this is the line that
> > causes my problems and needs to be transferred:
> > https://salsa.debian.org/debian/gmrender-resurrect/blob/master/debian/gmediarender.default#L8
> 
> Ah, I see, the problem is the $(hostname) part, which isn't supported by
> EnvironmentFile in systemd (which is the normal way to solve this
> problem).
> 
> After looking at this for a bit, my inclination if I were you would be to
> write a tiny shell script that loads /etc/default/gmediarender, constructs
> the command line, and then execs the daemon; install that script as
> /usr/share/gmediarender/start; and then invoke that script via
> start-stop-daemon in the init script and via ExecStart in the systemd unit
> file.  It's a little bit overkill because most of what the /etc/default
> file is doing is simple, but it looks like the easiest way to handle
> $(hostname).

I'd avoid using the wrapper for the init script TBH, because even
though that will make this a bit WET, it would otherwise make it more
complex there (having to use --startas and --exec to cope with the
intermediate interpreter usage).

I think shared wrappers make more sense when used to offload some kind of
pre/post "hook" work, which gets called from Exec{Start,Stop}{Pre,Post}
systemd service file directives (and init scripts).

But here you do have another option, but I'm not sure it might be
described as nicer TBH, :) something like this, or variations on this
theme:

  [Service]
  Type=simple
  EnvironmentFile=/etc/default/service-static-vars
  EnvironmentFile=-/run/service-dynamic-vars
  ExecStartPre=-/bin/sh -c 'echo NAME=$(hostname) >/run/service-dynamic-vars'
  ExecStart=/usr/bin/daemon --option ${NAME}

Thanks,
Guillem